feat: tlsping and tcpping using step-by-step (#815)

## Checklist

- [x] I have read the [contribution guidelines](https://github.com/ooni/probe-cli/blob/master/CONTRIBUTING.md)
- [x] reference issue for this pull request: https://github.com/ooni/probe/issues/2158
- [x] if you changed anything related how experiments work and you need to reflect these changes in the ooni/spec repository, please link to the related ooni/spec pull request: https://github.com/ooni/spec/pull/250

## Description

This diff refactors the codebase to reimplement tlsping and tcpping
to use the step-by-step measurements style.

See docs/design/dd-003-step-by-step.md for more information on the
step-by-step measurement style.

internal/testingx/time.go

@@ -0,0 +1,48 @@
+package testingx
+
+import (
+	"sync"
+	"time"
+)
+
+// TimeDeterministic implements time.Now in a deterministic fashion
+// such that every time.Time call returns a moment in time that occurs
+// one second after the configured zeroTime.
+//
+// It's safe to use this struct from multiple goroutine contexts.
+type TimeDeterministic struct {
+	// counter counts the number of "ticks" passed since the zero time: each
+	// call to Now increments this counter by one second.
+	counter time.Duration
+
+	// mu protects fields in this structure from concurrent access.
+	mu sync.Mutex
+
+	// zeroTime is the lazy-initialized zero time. The first call to Now
+	// will initialize this field with the current time.
+	zeroTime time.Time
+}
+
+// NewTimeDeterministic creates a new instance using the given zeroTime value.
+func NewTimeDeterministic(zeroTime time.Time) *TimeDeterministic {
+	return &TimeDeterministic{
+		counter:  0,
+		mu:       sync.Mutex{},
+		zeroTime: zeroTime,
+	}
+}
+
+// Now is like time.Now but more deterministic. The first call returns the
+// configured zeroTime and subsequent calls return moments in time that occur
+// exactly one second after the time returned by the previous call.
+func (td *TimeDeterministic) Now() time.Time {
+	td.mu.Lock()
+	if td.zeroTime.IsZero() {
+		td.zeroTime = time.Now()
+	}
+	offset := td.counter
+	td.counter += time.Second
+	res := td.zeroTime.Add(offset)
+	td.mu.Unlock()
+	return res
+}